Vertical seismic profiling (VSP) is a method of determining acoustic wave characteristics of rock layers in situ. The method includes lowering one or more sensors into a wellbore to a preselected depth. Typically several sensors are spaced apart to allow coverage over a preselected depth interval. A seismic signal is generated at or near the surface of the earth and propagates through the earth to be received by the sensors. These sensors convert the acoustic energy to sensing signals which are transmitted to the surface for suitable processing and recording.
U.S. Pat. No. 4,589,285 to Savit issued on May 20, 1996 discloses a vertical seismic profiling arrangement using optical fiber sensors. In particular, the disclosed system includes an elongated cable having a bi-directional optical fiber transmission link therein. A plurality of acousto-optic seismic sensors, each consisting of one- or multi-turn optical fiber coils, are coupled to the optical fiber transmission length by means of suitable directional optical couplers. The optical fiber coil making up each sensor acts as a resonant optical cavity to certain discrete wavelengths, as a function of the local static pressure environment within the borehole fluid. The resonant discrete wavelength under static conditions is the center or reference wavelength. Under dynamic conditions, the reference wavelength is data modulated (wavelength shifted) by transient pressure variations due to acoustic or seismic signals.
In the system disclosed in U.S. Pat. No. 4,589,285, each of the sensors are tuned to the same wavelength at the surface, and the varying static pressure, based upon the depth of a sensor, sets up a multitude of different wavelength carriers, each associated with a particular sensor, upon which acoustic seismic data can be superimposed. These different carrier wavelengths caused by the varying static pressure are intended to allow multiple sensors to exist on a single string without interfering with one another.
A problem associated with the VSP method disclosed in U.S. Pat. No. 4,589,285 is that the wavelength of any given sensor under static pressure conditions is unknown, and correlation of sensor signals to physical sensors, and therefore sensor depth, is difficult. Further, if the static pressure between two sensors is small, it may be difficult or impossible to differentiate between the signals arising from the individual sensors. Additionally, commercially available techniques for demultiplexing of wavelength division multiplex signals depends upon knowing the individual signal wavelengths and the channel spacing of the signals being received. Therefore, when using the disclosed method it is difficult or impossible to guarantee either the absolute wavelengths or spacings of the signals generated by the sensors. The sensors are exposed to the wellbore environment in order to obtain the wavelength shifts associated with the static pressure variation of the fluid column within the wellbore. However, the hostile high temperature and pressure and corrosive environment of an oil or gas wellbore may adversely affect the sensor string.
It is also known to perform distributed sensing utilizing optical Bragg grating sensors which are intrinsic to an optical fiber, see, e.g., U.S. Pat. Nos. 4,950,883; 4,996,419; 5,361,130; 5,401,956; 5,426,297; and 5,493,390. In such systems, Bragg gratings are utilized to form Bragg grating sensor strings, where each Bragg grating sensor produces a return signal having an optical bandwidth about a central wavelength (Bragg wavelength). The sensor string may be analyzed on a time division multiplex basis wherein return signals from various Bragg gratings in the sensor string are uniquely identified by their position in a pulse train of signals, such as disclosed in U.S. Pat. No. 5,361,130. Alternatively, as disclosed in U.S. Pat. No. 5,401,956, each Bragg grating sensor may have a central reflection wavelength different from that of the other fiber Bragg gratings such that the signals reflected by the Bragg grating sensor string are uniquely identified based on the wavelength of the received signals in a wavelength division multiplex system.
While such distributed fiber Bragg grating sensor systems have been utilized for distributed sensing of strain, temperature or other perturbations, such sensor systems have not been utilized for vertical seismic profiling in an earth borehole. In particular, as described above, an earth borehole of an oil or gas well presents an extremely hostile environment because of the high temperature, pressure and corrosive environment.
There therefore exists a need for an improved system for vertical seismic profiling of an earth borehole which provides highly accurate and reliable indication of seismic conditions while at the same time being resistant to the extremely hostile environment of an earth borehole.